The invention relates to a process for connecting a pipe in the region of its pipe end to a pipe coupling, comprising at least one support sleeve with an engagement means, wherein the pipe and the support sleeve are first pushed into one another, the pipe then being pressure deformed, thereby bringing about a form-fitting connection with the engagement means and bringing about a sealing relationship with sealing surfaces, mutually adjoining one another in axial direction. The invention relates furthermore to a connection brought about by the aforesaid process as well as to a pipe coupling suited therefor.
Pipe couplings are known for the connection of pipe ends of two pipes orxe2x80x94in the case of branchingsxe2x80x94of more than two pipes. The pipe couplings are so designed that they can be brought to overlap the pipes by sliding one into the other. In the overlapping region both sections are subjected to radial pressure deformation which results in bringing about a fluid impermeable connection between the pipe coupling and the pipes.
Depending on the material of the pipes, different pipe couplings are employed according to the state of the art. In the case of metal pipes, sleeve-like press fittings are used, comprising annular bulges at their ends projecting radially outwardly, into which sealing rings are inserted on the inside. Into the press fittings pipes are pushed from both sides. The annular bulges are then pressure deformed radially inwardly by plastic deformation both of the press fitting and also of the pipe, i.e. by means of a pressing device suited for this purpose. An example of such a connection is apparent from EP-A-0 198 789.
The use of sealing rings involves a number of drawbacks. On the one hand, there is no guarantee that the sealing ring remains intact when pushing the pipe into the press fitting, as its diameter is smaller than the outer diameter of the pipe. On the other hand, the chemical and thermal stability of such sealing rings over extended periods is not sufficiently ensured. As such pipe connections are employed in particular in the sanitary and heating sectors, presupposing a useful life of 50 years, high demands are made on the long-term stability of the elastomer used for the sealing ring. For heated and, in particular, chemically aggressive fluids, the working material of the sealing ring must be chemically and thermally resistant and must not lose its sealing capacity even if frequent temperature changes occur.
In order to avoid the use of sealing rings, a pipe connection is proposed in DE-U-94 19 106.9, wherein the press fitting is provided with an elastic plastics or rubber coating. For bringing about the connection, the pipexe2x80x94this may be a metal or a composite pipexe2x80x94is expanded and pushed over the press fitting. One or more annular beads are subsequently pressed into the pipe exterior so that the pipe material penetrates into the elastic coating of the press fitting. However, a connection of this type has only a limited useful life and is, above all, not suited for plastic pipes.
For connecting plastic pipes, made, for example, of crosslinked polyethylene or polypropylene, as well as of composite pipes, comprising successively from inside towards the outside a plastics layer, a metal tube or a metal coating andxe2x80x94as a protective coverxe2x80x94yet another plastics layer, two-part or three-part pipe couplings are proposed (EP-A-0 582 542, WO 92109840; DE-A-44 41 373; DE-C-196 27 608; JP-A-08 326 974). These pipe couplings include an interior component, from the central part of which one or more support sleeves extend, onto each of which a pipe can be pushed. Onto the pipe exterior a single pressure sleeve, engaging over two support sleeves (EP-A-0 582 543) is pushed, or separate pressure sleeves each (WO 92/09840; DE-A-44 41 373; DE-C-196 37 608; JP-A-08 326 974) are pushed onto the pipes and are then pressure deformed radially towards the interior, so that the terminal regions of the pipes between the pressure sleeve(s) and the support sleeves are in each case clamped over a relatively long distance. In order to improve the clamping action, grooves are moulded at least into the support sleeves xe2x80x94partly into the pressure sleeves as wellxe2x80x94, into which the plastics material of the pipes is pressed so that a wave-like pattern of the pipes in the clamping regions results, bringing about sealing surfaces adjoining one another in axial direction. In DE-A-196 37 608 specific embodiments of annular grooves in the support sleeve are proposed, into which the plastics material is pressed in order to subsequently bring about a sealing relationship within these grooves.
Such pipe couplings do not require additional sealing rings so that the above described problems do not occur here. On the other hand, their manufacture is more complex as their construction requires them to consist of a plurality of parts, necessitating additional processing steps in order to mould in the grooves. A considerable drawback resides further in the thermal performance of pure plastics or even composite pipes. It shows its negative effect in particular in the case of such pipe ducts which are exposed to considerable temperature fluctuationsxe2x80x94such as heating or hot water pipes. During heating, the plastics expands more than the metal of the pipe coupling, resulting in the occurrence of very high tensions in the regions of the clamped pipe, causing plastic flow, i.e., in particular, if the regions of the clamped pipe completely take up the space between the pressure sleeve and the support sleeve. The plastics then literally flows from these pipe regions and therefore out of the pipe coupling. In the course of subsequent cooling, the plastics contracts again, however, due to exceeding the flow limit, it no longer possesses the initial dimensions. The initial clamping pressure between the pipe and the pressure and support sleeve is no longer attained. If a relative movement has taken place between the pipe and the support sleeve, destruction of the plastics along the groove-like sealing points may occur as well.
Even more serious is the fact that the plastics may act like a kind of pump during the expansion and shrinking processes. Should a liquid film have formed in the gap between the upper limiting surfaces of the plastics and the support sleeve, liquid is locked in at indefinable points during heating and therefore expansion of the pipe. This, in turn, results in an increased expansion, as the volume of the enclosed liquid takes away space from the pipe sections clamped there. With repeated heating and cooling cycles, the plastics gradually looses its inherent tension and the original dimensions so that the tightness over extended periods is no longer ensured. One has therefore reverted to ensuring the tightness of the pipe connection by additionally providing sealing rings (WO 92/09840), having to accept once again their above described drawbacks.
When employed in drinking water supplies, a further problem resides in that no cavities must occur in the region of the pipe coupling, as small living organisms, such as germs or the like, may gather and live there. In pipe couplings taking the form of press fittings, in particular, such cavities exist between the press fitting and the pipe end regions. In addition, sealing rings become porous with time, which favours the presence of such living organisms. As far as plastics pipes are concerned, gaps and cavities come about by the above described expansion and shrinking processes, being in direct contact with the liquid. Furthermore, dissolved materials, such as for example magnesium or calcium carbonate, may also enter into the cavities and deposit there, thereby displacing the interface between the pipe end sections and the support sleeve. This promotes the penetration of water.
It is an object of the invention to so design a process of the type set out in the opening paragraph that reliable sealing between the pipe coupling and the pipes, in particular of plastics and composite pipes, is ensured in the long term. It is furthermore an object of the invention to provide a suitable connection therefor between the pipe coupling and the pipes as well as a pipe coupling therefor.
As regards the first part of the object, the solution according to the invention resides in that an axial force, bearing on the sealing surfaces, is applied to the pipe via a spring means, which takes support from the pipe coupling.
The basic concept of the invention is, therefore, the application of a defined axial force to the pipe, which presses the superposed sealing surfaces against one another, thus providing a reliable sealing relationship. It stands to reason that the axial force must be of such magnitude that the stress of the sealing surfaces is sufficiently maintained at all times under the operational conditions, expected to occur, and for the intended life span.
The advantage of the solution according to the invention, on the one hand, resides in that reliable sealing with high long-term stability is attained, even if the pipe is exposed to great temperature fluctuations, i.e. even if an additional sealing ring is dispensed with. On the side towards the fluid, only one sealing locality is necessary in principle so that no cavities form, into which water can enter.
It stands to reason that use is made of the teachings of the invention even if the process is used on a support sleeve in multiple successions, thereby attaining a redundant sealing relationship. However, one application of the process is normally sufficient. The process can in this context also be used for pipe couplings with branchings, having more than two support sleeves, since, what matters, is the application of the process to one support sleeve at a time.
The axial force application should preferably only be performed once a form-fitting connection between the engagement means and the pipe has been brought about, i.e. simultaneously with or after pressure deformation of the pipe.
The spring means does not have to be an additional component but can be brought about by the pipe itself in the course of pressure deformation of the pipe end by clamping a pipe section with the application of the axial force between the sealing means and a support means, provided in spaced apart relationship to the pipe end, and by providing the facility for radial expansion for the pipe on the side of the engagement means directed towards the pipe end. The clamping provides an axial force acting in the direction of the sealing means, in which context the pipe section, due to the facility to expand provided according to the invention, can yield in a flexibly resilient manner, preventing therefore a loss in mass on the pipe section. The spring action is therefore maintained to the extent intended, even during a change in temperature.
The abovementioned process is particularly suited for pure plastics pipes. This is so because the plastics material behaves in a particular way when the temperature increases, in as far as two opposing tendencies take effect. On the one hand, the material expands, bringing about a tendency to increase the axial force. However, on the other hand, the plastics material becomes soft at high temperatures with the tendency to decrease tension. On the whole, this results in a spring characteristic, preventing the formation of an excessive axial force at elevated temperatures, i.e. the axial force which initially increases because of the material expansion, is reduced again. This contributes to the fact that no material loss occurs beyond the engagement means.
Clamping is preferably performed in such a manner that the pipe is also put under pressure in the region of the support means in the direction towards the support sleeve by applying the axial force. The pressure deformation on both sides and the fixation of the pipe section by way of the engagement means and the support means causes the formation of a bulging pressure in the pipe section, resulting further in an arching of the pipe section with the formation of a spring action. In softer plastics pipes it is, moreover, advantageous if a wrap-around ring each is pushed over the pipe, at least in the region of the engagement means, and if the wrap-around ring(s) is/are likewise pressure deformed by plastic deformation.
The support meansxe2x80x94just like the engagement means as wellxe2x80x94may take the form of a tapered annular web so that it penetrates the pipe material during pressure deformation. It is, however, also possible to design the support means in such a manner that it acts from the end face of the pipe end. Examples of the different designs of support means are apparent from the embodiments illustrated in the drawings.
For pipes of little resiliency, such as for example composite pipes, rigid plastics pipes or metal pipes, it is advantageous to use an additional component to serve as the spring means and to bring the spring means into co-active engagement with the pipe after pushing the pipe onto the support sleeve by applying the axial force. This design allows composite pipes to have a substantially reduced wall thickness of the inner coating and to make it merely so thick as to render a form-fitting connection with the engagement means possible. In this context, contact with the aluminium pipe of the composite pipe should be avoided, so as to prevent intermetallic corrosion.
The active engagement between the spring means and the pipe can be performed in a multitude of manners, as shown by the working examples. It is particularly advantageous if the spring means, in order to bring about the co-active engagement, is pressure deformed, at least partly, in the direction of the support sleeve. This offers the advantage that sliding the pipe onto the pipe coupling can be performed without compression of the spring means. Only once the spring means has also been pressure deformed is the axial force applied, in which case pressure deformation may be performed in a single process step with pressure deforming the pipe for the purpose of bringing about the form-fitting connection, or it may be performed afterwards. The shape of the pressure jaw can ensure that pressure deformation of the spring means is not carried out too early. Pressure deformation of the spring means must, of course, be effected in such a manner as to still maintain its property as a spring, i.e. that it can yield resiliently.
Alternatively, a spring means is, however, also conceivable which comprises a pretensioned spring element and in which the spring element is released for the purpose of axial force application and this after the form-fitting connection between the pipe and the support sleeve has been brought about.
The action of the spring means may be performed from the end face of the pipe. It is, however, also possible to cause the spring means to act via the exterior or interior surface area of the pipe in that during and by the pressure deformation, a form-fitting connection between the spring means and the surface area is brought about. This is particularly advantageous for composite pipes, as in this manner intermetallic corrosion with metal parts of the spring means is avoided. Furthermore, it is not necessary to bring the pipe into contact with the spring means during insertion. This would involve considerable uncertainties. Precautions should, however, be taken that the pipe can expand towards the pipe end during an increase in temperature.
As regards the connection between the pipe and the pipe coupling, the object is attained in that at least one spring means is provided, deriving support from the pipe coupling by way of a support means, bringing about an axial force bearing on the sealing surfaces. As already described above with regard to the process according to the invention, this connection is characterised by reliable sealing with a long-term stability and by low manufacturing costs. Moreover, cavities are prevented, into which the fluid may seep.
In particular, in the case of pipes composed entirely of plastics, the spring means may take the form of a pipe section clamped between the engagement means and a support means, provided in spaced apart relationship towards the pipe end by applying the axial force and for which a space for radial expansion is provided. The pipe section may deform during temperature fluctuations because of the space for expansion provided according to the invention, so that its spring properties and thus also the axial force acting towards the sealing means, are maintained.
Clamping may be performed by pressure deforming the pipe even in the region of the support means in the direction towards the support sleeve, in which case advantageously pressure deforming is performed simultaneously in the region of the engagement means and the support means. In the process, the support means may likewise take the form of an engagement meansxe2x80x94preferably as a mirror-image to the engagement means bringing about the form-fitting connectionxe2x80x94, so that a form-fitting connection is brought about with the interior of the pipe. Alternatively, it may be provided that the support means takes the form of a support ring, from which the end face of the pipe end derives support after pressure deformation.
As already mentioned with regard to the process, the design of the spring means as an additional component, being in co-active engagement with the pipe while applying the axial force, is recommended for hard plastics pipes, composite pipes and metal pipes. The spring means ensures that even for that type of pipes an adequate axial force is applied under all intended operating conditions. In this context, in particular, at least one resilient tensioned spring element may be selected which is directly or indirectly connected to the pipe. This spring element may be a steel spring, for example in the form of a helical or disk spring, surrounding the support sleeve, or, alternatively, it may also be a plastics spring. Plastic elements, distributed over the circumference of the support sleeve may also be provided, instead of the plastics spring. The use of plastics for the spring element(s) has the advantage with regard to its spring characteristic, already mentioned further above, preventing the occurrence of an excessively high axial force at elevated temperatures. In the case of composite pipes, it is advantageous if the material of the plastics ring or plastics elements is identical with the plastics of the outer and inner coating of the composite pipe, i.e., in particular, crosslinked polyethylene.
Such a spring element is ideally tensioned by way of a pressure sleeve which has been plastically pressure deformed when bringing about the connection for the purpose of applying the axial force. In this process the pressure sleeve may extend over the pipe and may be pressed there onto the latter, preferably in the region of the engagement means bringing about the form-fitting relationship between the pipe and the support sleeve.
According to a further feature of the invention, provision is made for the interior surface area to comprise an axial support surface, directed towards the pipe end, onto which the spring means acts. The support surface is preferably moulded during pressure deformation to form a constriction, the spring element then extending into the pipe and being pressed onto the latter. This embodiment is suited primarily for metal pipes, in which case it is then advantageous to insert a sealing ring. One of the sealing surfaces may in this context be formed by the constriction while the other sealing surface may, for example, be designed as an annular web.
The spring means may, however, be designed as a spring sleeve as well, pressure deformed between the support at the pipe coupling and the pipe end, overlapping the pipe and pressure deformed with the pipe at that point as well.
The third part of the object according to the invention is characterised by a pipe coupling, wherein at least one spring means is provided, deriving support from the pipe coupling and adapted to be brought into co-active engagement with a pipe adapted to be pushed over the support sleeve. It is apparent from what is said further above how the spring means may be designed in detail. It is particularly advantageous, if the spring means is fitted to the pipe coupling in an axially immovable manner, thus forming part of the pipe coupling.